Monday, June 29, 2015

Green Algae Causes Leaf Spots on Avocados

Avocado trees, along with mangoes, can be attacked by a green algae called Cephaleuros virescens.  Most leaf spots are caused by fungi or bacteria. Occasionally, an alga will cause such spots with frequent rains and warm temperatures. The algae are dispersed by wind and splashing water. In addition to avocados and mangoes, about three hundred different plant species  are susceptible to this problem including guava, breadfruit, cacao, kava, tea (Camellia sinensis) and some citrus. 

Typically, green algae causes minor leaf spotting. Yet poor plant nutrition, poor soil drainage and stagnant air can predispose a tree to infection. On avocado and mango, the leaf spots appear burnt-orange to rust colored. On guava, the leaf spots are more brown or black, often with a yellow halo around each spot. The fruit can also be spotted, causing a scabby or scarring appearance, often affecting the marketability of the fruit. The algae do not cause a soft rot of the fruit. When attacking coffee, the algae is called ‘red rust’.

Again, with avocados and mangoes, there is generally no damage to the plant’s vigor or yield. Treatment is rarely needed.  Leaves on low hanging branches are mostly attacked. If desired, the low hanging branches that are affected may be pruned. Here are two recommendatons: keep the tree well fertilized and weeds under control. Copper fungicidal sprays, although usually not necessary, will control the disease.

Monday, June 22, 2015

Podocarpus Trees Attached by Beetles

The black twigborer (Xylosandrus compactus) is a beetle that attacks over 200 plant species including orchids, citrus, avocados, mango, coffee, eucalyptus, hibiscus and podocarpus. The females tunnel into woody twigs, leaving pin-sized entry holes. Once inside, they excavate galleries and lay eggs.  This excavation causes the damage to the tree. An infestation by one to three females is sufficient to kill the twig or branch. A severe infestation can kill a plant or even large trees. The damage is not caused by feeding, since the beetle larvae feed on a fungus that is introduced by the female beetle.

Typical symptoms of black twigborer are wilted and dead leaves beyond the beetle's entry hole. The dried, brown leaves frequently stay attached to the tree.

                                                    Entry hole

Cultural control
The best control is to maintain a vigorously growing tree. This will help the tree to resist beetle infestations and recover quickly from existing ones. Prune and destroy all beetle-infested plant material.

Monday, June 15, 2015

Corn Seedlings Hit the Ground

Don't think it's smooth sailing just because the corn seed has germinated and is up and growing.  Young seedlings are often found toppled over. There are three possible culprits for this dilemma.

The two most common offenders are birds and rats. Netting will help to exclude the birds and maybe a scarecrow, too.  Rats will come at night and nibble on the succulent seedlings. Bait or trapping is the solution for rats.

There are also some caterpillars like cutworms, that at night will feed on the seedlings, cutting off the stem at ground level. The plant is not consumed but damaged enough to cause the seedling to topple.

Removing old crop residue and surrounding weeds will reduce the cutworm population. Worms can be hand-picked from the ground at night. Spraying the seedlings and the ground with Bt (Bacillus thuringiensis), an organic pesticide,  may also help.

Photos: University of Minnesota Extension

Monday, June 8, 2015

Planting Trees At the Right Level

When planting trees, never plant them so high that the upper roots are exposed and susceptible to drying. On the other hand, never bury the trunk by planting too deeply; this can predispose the tree to disease. Some trees like citrus are quite susceptible to trunk or crown rot, while some species of trees are somewhat tolerant. Assuming the tree has originally been planted correctly in the container, maintain this same level.

Although a tree may be planted at the proper soil level,  it may later sink due to settling of the soil. What happens is an overzealous gardener will dig the planting hole deeper than the height of the container. The loosened soil that is put back can eventually settle, drawing the trunk down below the soil level. Essentially the planting hole should be as deep as required to accommodate the plant. The sides of the hole may be larger than the original container.  

When trees are near a sprinkler system, remember that the irrigation water should never be directed to hit the trunk of the tree. Again, some trees like citrus are quit susceptible to fungal diseases that occur when the trunk is wet, while other tree species may tolerate this condition.

Trees planted today may last a lifetime. Take the time to do it correctly!

Photo: Monkeypod tree, also known as Rain Tree (Samanea saman)

Monday, June 1, 2015

Lichens and Moss on Trees

Should I remove the moss and lichens growth that are growing on the trunk of my tree?

The light to dark greenish growth growing on the limbs and trunks of trees can either be moss or lichens. Lichens is a bit odd in that it is actually two different organisms, a fungus and an alga living together. It is the fungus that provides the shape with the algal cells growing in a layer near the top of the fungal structure. The algae manufacture food, a process called photosynthesis, for both the fungus and the algae: a symbiotic relationship.The lichen structure is usually grayish-green in color, paper-like, and forms small circular to irregular-shapes.

Lichen and moss are frequently seen growing on twigs and branches, especially under moist conditions. When wet from rain or dew, they are active growers but when dry, they stop growing. They do not die but lie dormant until the next rain starts.

 They are both considered harmless.  Although these organisms attach themselves to the tree, they are not pulling vital nutrients from that tree. It is often thought that lichens and moss are injuring a tree because they are growing so well and the tree is declining. There is a relationship between the vigorous growing moss/lichens and the decline of the tree, but not the way most people think. The trees are growing poorly not because of the moss or lichens, rather just the opposite. Trees decline first, shedding leaves and thus allowing more light to penetrate to the bark of the trunk and limbs. This, in turn, will cause a new growth spurt in the moss or lichens. This growth spurt is an indication that some other requirements of the plant are not being met. Check and make sure there is no insect infestations (scale, aphids), no old weed- whacker damage and make sure the tree is adequately fertilized. 

If you are troubled with this type of growth on your trees, take heart in the fact that most lichens will not grow in a smoky or polluted atmosphere.